In flat projection radiography examination types exist that aim at imaging a larger portion of the body than can be fit on a single, even the largest imaging cassette.
Such imaging needs arise in so-called full-leg or full-spine examinations, where clinical indication requires that e.g. the full spine or the full leg is examined at once so as to enable or to quantify diagnosis.
Analogous to conventional film-screen based radiography, in storage phosphor image acquisition wherein a radiation image is temporarily stored in a photostimulable phosphor screen, such examinations are achieved by filling a larger cassette with a plurality of storage phosphor screens, generally overlapping with one another so as to completely cover the longer, elongate cassette (an embodiment referred to as an overlapping sheet embodiment).
Such elongate cassettes are typically 35.times.105 cm for full-leg examinations and 30.times.90 cm for full-spine examinations. In the overlapping sheet embodiment, the largest single film cassette measures 35.times.43 cm and hence is unsuitable for imaging the spinal column or a leg. Typically, 4 screens are used, e.g. 4 (24.times.30 cm) screens are arranged in a 30.times.90 cm cassette, clearly resulting in an overlap configuration. Therefore, the part of the image corresponding to the overlap zone, will be less exposed.
Alternatively, the situation of non-overlapping images may also occur, e.g. in a configuration of 3 (35.times.35 cm) imaging screens in a 35.times.105 cm cassette, resulting in 3 pair-wise touching but otherwise non-overlapping images.
During exposure, a fixed grid is simultaneously present in the path of the X-ray beam, resulting in an image of a raster of horizontal and vertical parallel thin lines superposed to the radiation image of the elongate body. These lines aid the radiologist or operator in reconstructing the original geometry of the body, since lines on image must necessarily extend continuously and seamlessly into lines of the previous image and the next image.
In European patent application EP-A-97 200 857 and in U.S. Ser. No. 09/035,528, a method is disclosed to acquire a radiation image of an elongate body by using a sequence of recording members in partially overlapping disposition.
The image acquisition process assumes the use of an elongate cassette in which a sequence of stimulable phosphor screens are arranged in an overlapping manner so as to completely cover the cassette.
After exposure, the elongate cassette is opened, the individual screens are removed from the cassette and are each put into a smaller cassette of a size corresponding to the dimensions of an individual screen. Then, the cassettes are sequentially fed into a dedicated read-out apparatus where the cassette is again opened, the screen is taken out of the cassette, and scanned by means of stimulating radiation. The image-wise modulated light that is emitted upon stimulation is detected and converted into a digital image representation of part of the elongate body. Next, the digital image representations read out of each of the cassettes are recombined to form an image of the entire elongate body.
This patent application further relates to a method to obtain a single `stitched` image of an elongate body using the images read out of each of these recording members.
Patent applications EP-A-97 200 857 and U.S. Ser. No. 09/035,528 further disclose a method to obtain a `stitched` image of the elongate body, i.e. a method is disclosed for recombining the image representations read out of each of the cassettes into a complete image of the elongate body.
The method makes use of the image of a periodic grid which was placed on top of the elongate cassette in the path of irradiation so that it covers the full length of the cassette in a continuous way. The image of the grid was thus superimposed on the image of the elongate body which was recorded on the overlapping recording members. Since the grid covers the full length of the cassette (or of the elongate body) in a continuous way, its image should also be present in the resulting re-constructed image composition in a continuous way. This knowledge was used to enable alignment and stitching of the images with respect to each other.
The stitching method disclosed in above patent applications depends on two basic algorithms: (1) detection and modelling of the periodic grid and (2) cross-correlation of overlapping image parts. The disclosed stitching method deals with corrections for image distortions due to shift, overlap and rotation of a sub-image with respect to the previous and next sub-image.
It may however occur that the image of the attenuating grid or part thereof on a recording member is deformed relative to the original form of the grid. This means that parts of the image of the calibration grid are not congruent with the original physical grid. Since the image of the grid is used as a guidance for reconstructing the entire image of the elongate body from the partial images read out of the individual recording members, such deformation is unacceptable because it might lead to erroneous stitching.
Such problems have not been dealt with in the prior art teachings.